1. Field of the Invention
The present invention relates to a backlight unit to be used in liquid crystal displays, etc.
The invention also relates to a backlight unit in which the light source unit is filled with a transparent liquid.
The invention also relates to a reflector structure that realizes high-luminance and high-efficiency sidelight-type backlight units.
The invention also relates to a cold-cathode tube usable for a light source that receives essentially the fluorescence of the UV rays having been emitted through discharge emission of mercury or the like and emits visible light, especially for the light source of that type for liquid crystal displays.
2. Description of the Related Art
Recently, liquid crystal display panels have been rated highly in the market, as they save space upon installation and save power during operation, and their applications are expanding not only for displays of portable computers and monitors for portable televisions, but also for monitors of desk-top personal computers and flat televisions in domestic use. The backlight unit for lighting the liquid crystal display surface of such a liquid crystal display panel from the back surface of the panel includes two types; one being a direct-light-type unit that comprises a diffuser, a cold-cathode tube and a reflector all disposed just below the back surface of a liquid crystal display panel, and the other being a sidelight-type unit that comprises a diffuser, an optical waveguide and a reflector all disposed just below the back surface of a liquid crystal display panel, in which a cold-cathode tube and a reflector having a C-shaped or rectangularly U-shaped cross section are disposed on both sides of the optical waveguide.
For downsizing them and saving space upon installation, the latter is preferred to the former. However, the luminance of the former direct-light-type unit could be easily increased merely by increasing the number of the cold-cathode tubes in the unit, but it is difficult to increase the number of the cold-cathode tubes in the latter sidelight-type unit. It is therefore desired to increase the luminance of sidelight-type backlight units by increasing the emission efficiency of the units.
(Prior Art 1)
A sidelight-type backlight unit having a structure shown in FIG. 37A and FIG. 37B is generally used for liquid crystal display monitors. FIG. 37A is a view of a backlight unit of that type seen on its emission side. FIG. 37B is a cross-sectional view of FIG. 37A cut along the line A-A. As illustrated, the backlight unit comprises an acrylic plate 100 (this serves as an optical waveguide) with a light-scattering pattern 114 formed on its back surface, and two cold-cathode tubes 102, 104 disposed nearly in parallel with each other on and along one side of the acrylic plate 100. A reflector 110 (for this, an aluminum film is popularly used) is provided to surround the two cold-cathode tubes 102, 104, and its one side is opened to the optical waveguide 100 facing thereto. Also on and along the other side of the optical waveguide 100 having the two cold-cathode tubes 102, 104 disposed on its one side, other two cold-cathode tubes 106, 108 are disposed nearly in parallel with each other, and a reflector 112 is provided to surround the two cold-cathode tubes 106, 108 with its one side being opened to the optical waveguide 100 facing thereto.
In case where the number of the cold-cathode tubes in the sidelight-type backlight unit is increased for increasing the luminance of the unit, it produces some problems. One problem is the efficiency in light emission to the optical waveguide; and the other is the temperature of the cold-cathode tubes. Increasing the number of the cold-cathode tubes in the limited space in the unit inevitably makes the tubes more tightly adjacent to each other. As a result, in some region in the unit, the neighboring tubes will partly absorb the light emitted by them, thereby lowering the emission efficiency of the unit. In addition, in the area in which such an increased number of cold-cathode tubes are tightly aligned, the atmospheric temperature will increase, and if so, the tubes must be cooled so as to keep them at a temperature at which they ensure the maximum luminance.
In addition, the cold-cathode tubes in the unit involve by themselves a factor to lower the emission efficiency of the unit. As in FIG. 38, for example, the light emitted from one point of a cold-cathode tube 108 is partly reflected on the outer surface of the glass tube 136. In a cold-cathode tube having, for example, an outer diameter of 2.6 mm and an inner diameter of 2.0 mm, the reflected light accounts for at least 30% of the entire light emission from the tube. About 25% of the reflected light having reached the inner surface of the glass tube (for example, on the point c and the point d in FIG. 38) will be absorbed by the phosphor 138 coated on the inner surface of the glass tube or by the mercury gas filled in the glass tube. In addition, when the light from the cold-cathode tube 108 enters the glass tube of the neighboring cold-cathode tube 106, about 25% of the incident light that reaches the inner surface of the glass tube (for example, on the point a and the point b in FIG. 38) will be absorbed by the phosphor 138 coated on the inner surface of the glass tube or by the mercury gas filled in the glass tube.
To solve the prior art problems noted above, a method is proposed, which comprises filling the outer peripheral space of a cold-cathode tube with a liquid of which the refractive index is nearly the same as that of the glass material that forms the outer wall of the cold-cathode tube. According to this method, the reflection on the outer surface of the cold-cathode tube can be reduced, and, in addition, the incident light to the neighboring cold-cathode tube can be also reduced. Therefore, the method will be effective for increasing the emission efficiency of backlight units. In addition, since the liquid filled in the space around the cold-cathode tube will act also as a coolant, another advantage of the method is that the method does not involve the problem of temperature elevation even through a large number of cold-cathode tubes are packaged in the unit.
(Prior Art 2)
One conventional structure of a liquid crystal display with a sidelight-type backlight unit used therein is described, for which referred to is FIG. 41. As illustrated, a backlight unit is disposed adjacent to the emission side of a liquid crystal panel 134. The backlight unit is composed of a light source unit that comprises cold-cathode tubes (fluorescent tubes) 102 to 108 and reflectors 110, 112; and an optical waveguide unit that comprises a diffuser (optical sheet) 130, an optical waveguide 100 and a reflector 132. As the case may be, the diffuser 130 may have a multi-layered structure of plural sheets, depending on the mode of light diffusion through the optical waveguide unit.
For increasing the luminance of the backlight unit, two cold-cathode tubes of 102 to 108 are disposed for each of the reflectors 110, 112, and the optical waveguide 100 therefore has two pairs of cold-cathode tubes on both of its sides. The light emitted by the cold-cathode tubes 102 to 108 toward the optical waveguide 100 directly enters the optical waveguide 100 through its sides, and it is transmitted within the waveguide while being almost entirely reflected on and around it. The light emitted by the cold-cathode tubes 102 to 108 toward the reflectors 110, 112 is reflected by the reflectors 110, 112, and the thus-reflected light also enters the optical waveguide 100 through its sides and is transmitted within it like the direct light above.
Passing through the optical waveguide, a part of the light L1 goes out toward the reflector 132 or toward the diffuser 130, and the light that reaches the diffuser 130 passes through it while been diffused therethrough toward the liquid crystal panel 134. The light L2 that reaches the reflector 132 is reflected by it, and then passes through the optical waveguide 100 to reach the diffuser 130. This is also diffused toward the liquid crystal panel 134. In this manner, the liquid crystal panel 134 is illuminated by light diffused from two paths.
To meet the recent requirement for high-luminance backlight units, structures having a plurality of cold-cathode tubes disposed with one reflector are popular. In many cases, the shape of the reflector is determined depending on the external structure of the lighting unit and on the electric circuit and the wiring mode for the unit, for example, as in Japanese Patent Laid-Open No. 274185/1997.
(Prior Art 3)
An outline of the structure of the light source unit for conventional, direct-light-type backlight units is described with reference to FIG. 41 and FIG. 43. The structure of the direct-light-type backlight unit differs from that of the sidelight-type backlight unit shown in FIG. 41 in that, in the former, a plurality of straight light source tubes such as cold-cathode tubes 102a to 102d or the like are disposed below the diffuser 130 to be a surface light-emitting member and they are covered with a reflector 110 around them, as in FIG. 43; while in the latter, the optical waveguide 100 is disposed below the diffuser 130 and the light source units are on both sides of the optical waveguide 100, as in FIG. 41. The direct-light-type backlight unit is so constituted that the light emitted by the cold-cathode tubes 102a to 102d therein is, either directly or after having been reflected by the reflector 110, uniformly diffused through the diffuser 130, and then applied to the liquid crystal panel disposed adjacent to the unit.
For any of edge-light-type (sidelight-type) or direct-light-type backlight units, any of cold-cathode tubes 102, 102a to 102d, and 104 to 108 of the same type are used. The cold-cathode tube is made of a glass tube 136 with an electrode fixed on both of its sides, and the inner surface of the glass tube 136 is coated with a phosphor 138. Mercury, argon and neon are sealed in the glass tube 136. For the glass tube 136, generally used is hard glass having a refractive index of 1.5 or so.
When an electric current is applied between the two electrodes fixed on the glass tube 136, the mercury gas sealed in the glass tube 136 is excited, and radiates UV rays (essentially UV rays having a wavelength of 185 nm or 254 nm). The phosphor 138 coated on the inner surface of the glass tube 136 absorbs the UV rays, and emits visible light. The visible light is radiated outside the glass tube 136, and is utilized for illuminating liquid crystal panels.
(Prior Art 4)
A conventional cold-cathode tube serving as a light source that receives essentially the fluorescence of the UV rays having been emitted through discharge emission of mercury or the like and emits visible light, for example, that for a light source for liquid crystal displays and others is described with reference to FIG. 44A and FIG. 44B. For the light source for liquid crystal displays, cold-cathode tubes coated with phosphors capable of emitting light of three primary colors are used. For ordinary cold-cathode tubes, a phosphor mixture prepared by mixing (SrCaBa)5(PO4)3CL:Eu, LaPO4:Ce,Tb, Y2O3:Eu and the like in a predetermined ratio is baked on the inner surface of the glass tube 136, as in FIG. 44A. The phosphors are white translucent powders, and they are fixed on the inner surface of the cold-cathode tube generally via a binder consisting essentially of water glass. Cold-cathode tubes of that type, reflectors (essentially made of aluminium) to surround them, and a tabular optical waveguide (acrylic plate) are assembled into a backlight unit such as that shown in FIG. 37A and FIG. 37B, and the unit is disposed behind a liquid crystal panel.
(Prior Art 5)
A surface light source unit having electric discharge tubes therein is grouped into two types, one being a direct-light-type unit and the other being a sidelight-type unit, as so mentioned hereinabove. However, the structures of these types illustrated in FIG. 37A through FIG. 41 and FIG. 43 are problematic in that they could hardly satisfy all the requirements for overall thickness reduction, uniform light diffusion and increased luminance. Specifically, the direct-light-type unit can realize increased luminance relatively with ease, but could hardly ensure uniform light diffusion owing to the luminance difference between the area around the discharge tubes and the area remote from the discharge tubes. In addition, since the discharge tubes are disposed below the light-emitting member therein, the overall thickness of the direct-light-type unit is difficult to reduce. Moreover, the positional relationship between the discharge tubes and the light curtain disposed between the diffuser and the discharge tubes is a matter of great importance to the direct-light-type unit, but it is difficult to appropriately align them in every unit. For these reasons, direct-light-type units actually produced on an industrial scale often involve the problem of luminance fluctuation among them.
On the other hand, the sidelight-type unit can be thinned with ease and can ensure uniform light diffusion also with ease, but its luminance is difficult to increase since the incident light utilization in the optical waveguide therein is low. To solve the problem, Japanese Patent Laid-Open No. 248495/1995 discloses a backlight unit of a different type as in FIG. 45. As illustrated in FIG. 45, the backlight unit has a UV lamp 300 partly covered with a reflective film 308, and has a dichroic mirror 304 disposed between the UV lamp 300 and an optical waveguide 302. In this, the mirror 304 faces the UV lamp 300; and a phosphor film 306 is laminated on the mirror 304, and this faces the optical waveguide 302. Owing to its wavelength selectivity, the dichroic mirror 304 disposed in this unit can pass substantially UV rays only through it, and it greatly improves the luminescent light utilization efficiency of the unit.
In the prior art 1, the method of filling the outer peripheral space of a cold-cathode tube with a liquid of which the refractive index is nearly the same as that of the glass material that forms the outer wall of the cold-cathode tube is problematic in that the light diffusion through the optical waveguide is not good. FIG. 39 shows a backlight unit in which the outer peripheral space of each cold-cathode tube is filled with a liquid, and this is seen in the same direction as that for the view of FIG. 37B. In FIG. 39, the same constituent members as those in FIGS. 37A and 37B are designated by the same numeral references as therein. The light source unit (composed of the cold-cathode tubes 102, 104 and the reflector 110) is filled with a transparent liquid 116 of which the refractive index is nearly the same as that of the glass tube for the cold-cathode tubes 102, 104, and is connected with the optical waveguide 100 via an optical adhesive 120 therebetween. The same shall apply to the light source unit (composed of the cold-cathode tubes 106, 108 and the reflector 112) on the opposite side.
In this structure, however, the part extending from the cold-cathode tubes 102, 104 to the optical waveguide 100 form a substantially continuous body. In this part, therefore, the optical waveguide 100 will lose the waveguide condition for it (the condition is that, in principle, all the light from the cold-cathode tubes entirely enters the optical waveguide 100 on its side surface at an incident angle larger than the critical angle thereto). By way of example, a light source unit of FIG. 40 is referred to. In the case where the optical adhesive 122 and the transparent liquid 118 are not present in the unit, for example, the light from the cold-cathode tube 106 shall be refracted at one end of the optical waveguide 100 to run in the refracted direction of the dotted line P. With that, the thus-refracted light will run through the optical waveguide 100 while undergoing repeated total reflection therein. However, in case where the refractive index of the members that form the optical path is unified by the optical adhesive 122 and the transparent liquid 118, the light from the cold-cathode tube could not be refracted but shall go straight ahead as in the solid line Q, and it will be out of the optical waveguide 100.
Next discussed hereinunder are the problems with the prior art 2 and the prior art 3. The problem with the liquid crystal display panel equipped with a backlight unit of FIG. 41 is analyzed with reference to the view of FIG. 42. FIG. 42 shows the right-side light source unit of the structure of FIG. 41. Of the light having been emitted by the cold-cathode tube 102, the light m1 running toward the optical waveguide 100 directly enters the optical waveguide 100 through its end. The light m2 running toward the reflector 110 opposite to the optical waveguide 100 is reflected by the reflector 110, and then enters the optical waveguide 100 through its end.
However, the light m3 that is reflected by the reflector 110 and again enters the cold-cathode tube 102, and the light m4 that directly enters the neighboring cold-cathode tube 104 will be absorbed by the phosphors existing in the cold-cathode tubes 102, 104 or will be multi-reflected indifferent directions by the glass that forms the cold-cathode tubes, depending on the incident angle of these rays m3 and m4 entering the cold-cathode tubes 102, 104. As a result, some light emitted by the cold-cathode tubes could not enter the optical waveguide 100. Even if the light having entered the cold-cathode tubes 102, 104 could be again emitted from them, it will be again reflected by the reflector 110 and will further again enter the cold-cathode tubes 102, 104, and, after all, the light will be significantly attenuated. For these reasons, the light emitted by the cold-cathode tubes 102, 104 could not be efficiently utilized in the unit, thereby causing the problem of the reduction in light emission efficiency of the unit and the problem of the insufficiency of luminescent light quantity in the unit.
To increase the light quantity in the unit, increasing the number of cold-cathode tubes therein and increasing the electric power to be applied to the cold-cathode tubes may be taken into consideration, which, however, will produce still other problems. Increasing the number of cold-cathode tubes will inevitably enlarge the overall size of the lighting unit; and increasing the electric power to be applied to the cold-cathode tubes will increase the quantity of heat to be generated by the light source and will increase the light emission noise of the cold-cathode tubes.
Japanese Utility Model Laid-Open No. 59402/1993 and Japanese Patent No. 2,874,418 have proposed a technique of optimizing the shape of reflectors for direct-light-type backlight units. In direct-light-type backlight units, however, the reflector must produce uniformly reflected rays that are parallel with each other. Therefore, the proposed technique is problematic in that that the intended optimization is limited as it must satisfy the requirement as above and must increase the reflector efficiency. As opposed to the direct-light-type backlight units, sidelight-type backlight units could easily solve the problem since the light emitted by the cold-cathode tubes therein may be directly led into the optical waveguide. However, owing to the limitation on the thickness of the lighting unit, the diameter of each cold-cathode tube must be at most 3 mm, preferably 2.6 mm or so, relative to the aperture of the reflector (in general, it is at most 10 mm and is preferably 8 mm or so). Therefore, the method of increasing the number of cold-cathode tubes in sidelight-type backlight units is limited, and increasing the luminance of the units is therefore difficult.
In addition, the above-mentioned prior art techniques involve still another problem in that the luminous efficiency (light emission efficiency) of the cold-cathode tubes employed therein is only 30 lumens/W of the inputted power, and is extremely small.
Next discussed is the problem with the prior art 4. In the structure of FIG. 44A, the emission efficiency will lower when the visible light is emitted out of the cold-cathode tube. The reason is because a gaseous (or vacuum) space 202 is formed between the powdery phosphor particles 200 and the glass tube 136, as in FIG. 44B. When the visible rays emitted on the surfaces of the phosphor particles have reached the glass tube 136, some of them are reflected on the surface of the glass tube like X1, while some others pass through the glass tube like X2. Since the glass material to form the cold-cathode tube generally has a refractive index of 1.48 or so, the surface reflection X1 causes a reflection loss of around 10%.
In this connection, analyzed is a case where some external visible light enters the cold-cathode tube, with reference to FIG. 44B. When light (designated by solid lines in FIG. 44B) enters the glass tube 136 through its outer surface (this is on the lower side in FIG. 44B), the incident light is reflected on the surfaces of the phosphor particles 200 that are in contact with the space 202. In this case, however, since the surfaces of the particles are not smooth and since the diameter thereof is 3 μm or so and is small, the reflected light shall be macroscopically considered as scattered light. Therefore, the light passing through the cold-cathode tube or reflected by the phosphor particles will lose its running orientation, and will be thereby diffused and reflected as in the manner designated by the dotted lines in FIG. 44B. As a result, in the lighting unit with conventional cold-cathode tubes therein, the light having entered the cold-cathode tubes shall be lost. The light loss increases to a higher degree in more small-sized lighting units. In current backlight units, about 60% of the overall light emission will re-enter the cold-cathode tubes, and 30% of the light having re-entered them (this corresponds to about 18% of the overall light emission) will be scattered or absorbed by the phosphors and will be thereby lost.
Next discussed is the problem with the prior art 5. Even in the structure of FIG. 45, a part of UV rays having been emitted by the UV lamp 300 will be multi-reflected in different directions in the UV lamp 300 and will be absorbed by the gas existing therein. Therefore, the problem with the structure is that the quantity of UV rays to be emitted outside by the UV lamp decreases and the emission efficiency of the structure could not be increased. In addition, when the gas in the UV lamp absorbs too much light, the temperature of the UV lamp rises. Therefore, the size of the UV lamp could not be reduced. Another problem with the structure is that the light loss therein is great since the light emitted by the UV lamp is scattered in the UV lamp and is absorbed by the gas existing therein.
In the sidelight-type backlight unit, optical elements that may disorder the waveguide condition, such as the diffusive surface of the diffuser 130 and the refractive and reflective surface of the reflector 132, may be disposed in any desired density, whereby the quantity distribution of the light that passes through the optical waveguide 100 can be controlled, and the backlight unit ensures illumination of extremely high uniformity. In addition, the backlight unit of the type is characterized in that even when some of the light-emitting surfaces of the cold-cathode tubes 102 to 108 are aged so that the light emission through them is lowered, the unit could seemingly emit uniform light since the distance between the cold-cathode tubes and the panel surface to be illuminated by the unit is long. On the contrary, however, since the cold-cathode tubes 102 to 108 are disposed adjacent to the side edges of the optical waveguide 100 in the backlight unit of the type, the number of the cold-cathode tubes that may be in the unit is limited. Therefore, one problem with the unit of the type is that it is difficult to increase the luminance of the unit.
On the other hand, the direct-light-type sidelight unit is advantageous in that its luminance can be increased by increasing the number of the cold-cathode tubes 102a to 102d, but is problematic in that its luminance is often uneven since the distance between the cold-cathode tubes 102a to 102d and the panel surface to be illuminated by the unit is not long. It may be possible to optimize the distance between the cold-cathode tubes 102a to 102d, the characteristics of the diffuser 130 and the profile of the reflector 110 to thereby evade luminance fluctuation. However, when the conditions are varied, some of the light-emitting surfaces of the cold-cathode tubes 102a to 102d will be aged to lower the light emission through them, and one problem with the unit of the type is that its luminance will readily fluctuate.